The present invention relates to a linear actuator for causing two air cylinder mechanisms mounted in a pedestal to operate synchronously to cause a slide table on the pedestal to operate linearly and more specifically relates to a linear actuator having means for stopping the slide table at a stroke end in a cushioned manner.
As disclosed in Japanese Patent Application Laid-open No. 10-61611, for example, there is a known linear actuator having two air cylinder mechanisms mounted in a pedestal and causing the air cylinder mechanisms to operate synchronously to cause a slide table on the pedestal to reciprocate linearly.
In such a linear actuator, various cushioning mechanisms are attached for stopping the slide table at a stroke end in a cushioned manner. For example, a damper elastically biased by a spring is provided to a side face of the pedestal and a contact member provided to a side face of the slide table is brought into contact with the damper at the stroke end.
However, the cushioning mechanism provided to any known linear actuator mechanically absorbs a shock, has a simple structure, and its operation is reliable, but cannot be used for some uses because a sound of the shock is produced or the cushioning mechanism projects from a side face.
On the other hand, in a normal air cylinder device, a cushioning mechanism of an air cushion type is used in which air is temporarily sealed in pressure chambers on an exhaust side in operation of pistons to increase pressure of the pressure chambers and to decelerate the pistons by the exhaust pressure, thereby causing the pistons to stop at the stroke ends in the cushioned manner.
However, because a long cushion ring is provided on at least one side of the piston and a long cushion chamber into which the cushion ring is fitted is provided in the pressure chamber in the above cushioning mechanism of the air cushion type, the length in an axial direction of a cylinder increases and the size of the linear actuator is increased if the cushioning mechanism is applied to the linear actuator. Furthermore, because the linear actuator has two air cylinder mechanisms, the size of the linear actuator is further increased if the air cushion is provided for each air cylinder mechanism.
It is a technical object of the present invention to provide a linear actuator having a small and rational design structure including a cushioning mechanism of an air cushion type.
To achieve the above object, a linear actuator of the invention comprises two air cylinder mechanisms which are arranged in parallel with each other and operate synchronously, a pair of ports common to both the air cylinder mechanisms, and at least one air cushion mechanism common to both the air cylinder mechanisms, wherein the air cushion mechanism has an exhaust hole which is provided to a position adjacent to at least one of the ports and which communicates with the pressure chambers at positions closer to chamber ends than the ports, a flow rate restricting mechanism which is connected between the exhaust hole and the port and which restricts a flow rate of exhaust discharged from the pressure chambers, and cushion packing which is mounted to an outer peripheral face of one of the pistons and which gets over the port on an exhaust side immediately before the piston reaches the stroke end to cause the compressed air in the pressure chambers to be discharged only from the exhaust hole.
In the linear actuator of the invention having the above structure, if the compressed air is supplied to or discharged from the pressure chambers of the respective air cylinder mechanisms through the pair of ports, the pistons of both the air cylinder mechanisms operate synchronously and a slide table reciprocates linearly on a pedestal.
Stopping of the slide table in a cushioned manner when the slide table reaches the stroke end is carried out by synchronously decelerating the pistons of the two air cylinder mechanisms by the common air cushion mechanism.
In other words, in sliding of the pistons of the respective air cylinder mechanisms, the compressed air in the respective pressure chambers on the exhaust side is discharged at first mainly through the port. When the piston approaches the stroke end and the cushion packing gets over the exhaust-side port, the port is separated from the pressure chambers and the compressed air in the pressure chambers is discharged only from the exhaust hole through the flow rate restricting mechanism in a restricted manner. As a result, the pressure in the pressure chambers is increased by control of the flow rate by the flow rate restricting mechanism and the increased pressure functions as piston back pressure to decelerate the pistons while causing the pistons to reach the stroke ends.
As described above, because the linear actuator has the cushioning mechanism of the air cushion type, the linear actuator does not produce a sound of a collision and is quiet unlike a mechanical cushioning mechanism.
The linear actuator does not produce dust and can be used in a clean room and the like. If the mechanical cushioning mechanism is provided to only one side of the slide table like in prior art, the slide table is supported on the one side when it stops and therefore, an axis of the slide table is likely to incline. In the present invention, however, a cushioning effect acts on the pistons of the respective air cylinder mechanisms coaxially with a direction in which thrust of the pistons is produced and the air cylinder mechanisms are synchronously decelerated. Therefore, inclination of the slide table is not generated. Furthermore, not only because the one air cushion mechanism common to the two air cylinder mechanisms is provided but also because the air cushion mechanism does not require a long cushion ring and a long cushion chamber into which the cushion ring is fitted unlike the prior art, it is possible for the linear actuator to be very small and have a rational design structure.
According to a concrete embodiment of the invention, the flow rate restricting mechanism includes a throttle hole for restricting a flow rate of exhaust flowing from the exhaust hole toward the port and a check valve for restricting a flow of exhaust from the exhaust hole toward the port and for allowing a flow of compressed air in a reverse direction.
In this case, it is preferable that a valve chamber communicating with the exhaust hole and the ports is formed in the pedestal and the flow rate restricting mechanism is mounted into the valve chamber by disposing a valve member having the throttle hole in the valve chamber through a lip seal forming the check valve.